The invention relates to a process for the separation of isomeric dichlorotoluenes by absorption on zeolites. Dichlorotoluenes are generally prepared by chlorination of toluene or of monochlorotoluenes in the presence of a Lewis acid such as the trichlorides in the presence of a Lewis acid such as the trichlorides of aluminum, iron or antimony, by themselves or together with a cocatalyst such as, for example, sulphur or sulphur chlorides.
The chlorination reaction leads to mixtures containing the 2,4-, 2,5-, 2,6-, 3,4- and 2,3-dichlorotoluenes. These mixtures can be separated by distillation of the other chlorination products of toluene or of the monochlorotoluenes (monochlorotoluene and trichlorotoluenes). The five isomers mentioned above are obtained in varying proporations depending on the starting products used for the chlorination reaction.
It is also possible to separate the mixtures of dichlorotoluenes by distillation into two fractions boiling at about 201.degree. C. and about 209.degree. C. The first fraction comprises the 2,6-, 2,4-, and 2,5-isomers and the second fraction is composed of the 3,4- and 2,3- isomers.
It is generally accepted that it is not possible to obtain all the various isomers in a pure state under economically acceptable conditions by conventional techniques of distillation or of fractional crystallization. In particular, distillation does not allow the separation of the constituents of the two fractions boiling at 201 and 209.degree. C. approximately, because of the very slight differences in boiling point of the isomers. Only 2,3-dichlorotoluene can be separated off by distillation, provided that o-chlorotoluene is used as the starting material. As regards fractional crystallization, it can generally not be used because numerous eutectic mixtures exist.
In view of these considerations, other separation techniques have been proposed. In particular, U.S. Pat. No. 4,254,062 describes a process for the separation of isomeric dichlorotoluenes which employs zeolites of types X or Y.